How long does an engine last provided we do three-month interval oil changes with bulk synthetic oil at Walmart?

Hi, we have the vehicle listed above SV with a 1.6-liter twin cam or dual overhead camshaft variable valve timing engine that we have been maintaining diligently meticulously and painstakingly at Walmart with three-month interval bulk synthetic oil changes. My question is how long can this engine last and what is variable valve timing? I have heard that variable valve timing has an electronic variable valve timing actuator and a variable valve timing gear in front of the exhaust camshaft but how does variable valve timing vary the opening and closing of the intake and exhaust valves, and it is synchronized with which internal combustion engine stroke? The intake stroke? The compression stroke? The power stroke? The exhaust stroke? Also, I saw in the Hays Repair Manual that this engine has an oil pump chain at the bottom and a timing chain at the top, does this mean that if the timing chain stretches then oil pressure will be reduced along with the compression from the stretched or jumped timing chain? Thank you in advance for your valuable information as the Hays Repair Manual is way too technical and I am having trouble reading it and that's why I am turning to your website for automotive information. Written by Alex Stathas